Driver Method

ABSTRACT

The invention provides advances in the arts with useful and novel driver methods. The invention provides circuit driver and control methods for relatively high-current drivers, usable with relatively low-voltage battery power sources. Preferred embodiments include one or more high series resistance capacitors electrically connected with a power source. A low resistance driver circuit regulates power supplied from the capacitors to the load.

PRIORITY ENTITLEMENT

This application is entitled to priority based on Provisional PatentApplication Ser. No. 61/382,747 filed on Sep. 14, 2010, which isincorporated herein for all purposes by this reference. This applicationand the Provisional patent application have at least one commoninventor.

TECHNICAL FIELD

The invention relates to microelectronic driver methods and associatedcircuitry. More particularly, the invention relates to driver methodsparticularly suited for use with flash driver circuits and systems fordriving LEDs using capacitors as a power source.

BACKGROUND OF THE INVENTION

It is sometimes desirable to use components with high currentrequirements in portable electronic apparatus. Problems arise, howeverwith driving high-current devices using relatively low voltage powersupplies, such as common batteries for example. On the one hand, batteryvoltage must be sufficient to drive the high-current devices. On theother hand, the current requirements may be so high that there is a riskof damaging the batteries. An example is the use of powerful LEDs asflash elements in small cameras. Overall, this is desirable in order toreduce battery drain, reduce cost, and minimize device size compared toxenon flash systems. Commonly available Lithium Ion (Li-Ion) batteriesoften used in such applications are limited in their voltage capacities,however, and are often incapable of withstanding the high currentsrequired for driving the LEDs. One potential solution to the problem isto use the available batteries for charging capacitors capable of beingcharged to sufficient voltage levels and then in turn using thecapacitors to drive the LEDs. In such instances it would be desirable touse a low-resistance switch to drive the LEDs if not for the followingproblems. The current has a tendency to change as the capacitors aredischarged. The LEDs themselves can have significant processingvariation, in turn yielding forward voltage variation. There is also atemperature coefficient associated with the forward voltage drop of theLEDs, which affects the charging performance.

Due to the foregoing and other problems and potential advantages,improved driver methods, particularly LED driver methods, would beuseful contributions to the applicable arts.

SUMMARY OF THE INVENTION

In carrying out the principles of the present invention, in accordancewith preferred embodiments, the invention provides advances in the artswith useful and novel methods for driving circuits using improvedtechniques for changing drive current and voltage levels as needed in aparticular application. All possible variations within the scope of theinvention cannot, and need not, be shown. It should be understood thatthe invention may be used with various conductive materials and varioussubstrate materials, components, and IC package formats. It should beunderstood that the drive current control techniques described hereingenerally imply analogous drive voltage techniques, and vice versa.

According to one aspect of the invention, in an example of a preferredembodiment, a method is disclosed for driving a load using a step ofdetermining a drive current requirement associated with the load. In afurther step, a voltage level sufficient to produce the drive currentrequired by the load is determined. Another step includes providing therequired drive current to the load.

According to another aspect of the invention, a preferred embodiment ofa driver method includes steps for characterizing load elements andderiving in advance voltage levels required for producing and providingthe required drive current for the load.

According to another aspect of the invention, a preferred embodiment ofa method for driving a load includes steps for determining the requiredload current and deriving a corresponding voltage level, in part bymeasuring equivalent series resistance of a circuit for providingcurrent to the load.

According to another aspect of the invention, a preferred embodiment ofa method for driving a load includes steps for providing one or morecurrent pulses to the load in order to determine the required loadcurrent and steps for using the one or more current pulses to calculatethe drive current requirement.

According to another aspect of the invention, a preferred embodiment ofa method for driving a load includes steps for extracting feedback fromthe load for use in the deriving the appropriate voltage level forproviding the current required by the load.

According to another aspect of the invention, a preferred embodiment ofa method for driving a load includes steps for storing and/or retrievinghistorical data relative to drive current in order to determine drivecurrent requirements.

The invention has advantages including but not limited to providing oneor more of the following features, reduced driver dropout, reduced powerdissipation, and improved efficiency. These and other advantageous,features, and benefits of the invention can be understood by one ofordinary skill in the arts upon careful consideration of the detaileddescription of representative embodiments of the invention in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from considerationof the description and drawings in which:

FIG. 1 is a process flow diagram illustrating examples of preferredembodiments of driver methods in accordance with the invention;

FIG. 2 is a simplified schematic circuit diagram illustrating an exampleof preferred embodiments of methods and associated circuits and systemsand according to the invention; and

FIG. 3 is a simplified schematic circuit diagram illustrating an exampleof preferred embodiments of methods and associated circuits and systemsand according to the invention.

References in the detailed description correspond to like references inthe various drawings unless otherwise noted. Descriptive and directionalterms used in the written description such as front, back, top, bottom,upper, side, et cetera, refer to the drawings themselves as laid out onthe paper and not to physical limitations of the invention unlessspecifically noted. The drawings are not to scale, and some features ofembodiments shown and discussed are simplified or amplified forillustrating principles and features as well as advantages of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Although the making and using of various specific exemplary embodimentsof the invention are discussed herein, it should be appreciated that thesystems and methods described and shown exemplify inventive conceptswhich can be embodied in a wide variety of specific contexts. It shouldbe understood that the invention may be practiced in variousapplications and embodiments without altering the principles of theinvention. For purposes of clarity, detailed descriptions of functions,components, and systems familiar to those skilled in the applicable artsare not included. In general, the invention provides improved drivermethods. Preferred embodiments of the invention include dynamicadjustment of driver voltages and/or currents according to real timeconditions. The invention is described in the context of representativeexample embodiments. Although variations in the details of theembodiments are possible, each has advantages over the prior art.

An approach used in the methods of the invention is to adapt the drivervoltage supply output to meet the need for current at the load beingdriven. The technique used by the invention includes endeavors to setthe voltage at a level slightly higher than that required to support thedesired load current. As a result, the load may be driven with arelatively low dropout. In turn, the power dissipation and efficiency ofthe entire system driving the load may be improved by reducing thedriver dropout and therefore the power dissipation in the driver.

The invention provides several alternative approaches which may be usedto detect and set the correct voltage for providing the required loadcurrent. Referring initially to FIG. 1, the flow of a method 10 ofdriving a load is shown. Preferably, the load is monitored 12 for itscurrent and voltage characteristics. In some instances, it may bedesirable to monitor additional parameters, such as temperature orlight, for example. From the monitored load, a drive current suitable todrive the load is determined 14. Based on the drive current requirement,the voltage level sufficient to produce the drive current may bedetermined and provided 16. The voltage level may be derived from themonitored drive current level and any additional offsets required basedon the characteristics of the load circuit, including the power source,expected losses due to resistance, temperature, and similar factorsaffecting circuit performance. For efficiency, the required load currentis preferably provided using a circuit having low resistance. There area number of variations possible in performing the steps indicated. Insome applications, it may be deemed sufficient to determine the drivecurrent in a single instance, using the determination for all futureload events. In other applications it may be more desirable to determinethe load current periodically, or on an event-by-event basis in realtime. Further examples are described herein.

In an example of a preferred method for controlling a driver circuitwhich obtains its power from one or more storage capacitors, theequivalent series resistance (ESR) of the storage capacitors introducean additional variable to the operation of the circuit. Accordingly, itis preferred to monitor the ESR of the capacitors as they are charged.The information obtained from this monitoring is then used to calculatean appropriate adjustment to be included in the supply rail set point.In order to measure the ESR, the voltage during charging is measured ata known charging current. The charging is stopped at a given time andthe capacitor voltage is measured again. The ratio of the voltage to thecharging current is used to calculate the capacitor ESR. Knowing the ESRand the required load current, e.g., the flash current in a flash LEDcircuit, the LED drive voltage is then increased accordingly. Forapplications in which the capacitor charging current is different fromthe LED flash current, an additional multiplier can be determined foruse in deriving the proper relationship.

As shown in each of FIGS. 2 and 3, in examples of systems forimplementing the methods of the invention, a circuit is provided inwhich a battery is coupled to a step-up converter for charging aplurality of capacitors. The capacitors are coupled in series with aload device, in this example LEDs, having fairly high voltage andcurrent requirements relative to the battery. The capacitors areselected for their ability to provide a relatively high current pulse atthe LEDs without overtaxing the battery. In this example assuming an LEDrequiring about 4V, it is preferred to place at least two capacitors inseries in order to provide enough supply voltage to be able to driveeach LED. In principle, any number of capacitors may be placed inseries, but on the other hand, it is desirable to minimize the number oflarge capacitors that must be used in a system. As further describedherein, it is preferred to use various sensors and controls, such asthermal and/or image sensors, and PWM and/or microcontrollers, tocontrol the operation of the systems used to implement the methods.

In an example of a preferred method for controlling a driver circuitwhich is anticipated to vary in its operational parameters, it may bepreferable to characterize the system in real time in order to determinethe appropriate supply voltage for ensuring the appropriate drivecurrent. In order to do so, the load is pulsed one or more times priorto an actual flash event. Pulses of one or more different voltages closeto or approaching that anticipated to be required to obtain the desiredcurrent level may be used. The current and voltage of each of the pulsesis then used to derive the voltage required to provide the desiredcurrent for driving the load. An example of an implementation of thismethod is to continuously flash an LED, or array of LEDs, duringcharging of associated storage capacitors. The capacitors continue tocharge until the LED drive voltage is sufficiently high to achieve therequired current output. In the example of an LED flash application, inorder to minimize the visible impact of the pulsing calibration, one ormore low voltage and low current pulses may be used. The light given offby the LEDs due to a low current pulse is negligible. In this example,since the current is much lower than that required for a flash event,additional extrapolation is required to derive the desired supplyvoltage. This extrapolation may be linear, piece-wise-linear,exponential, lookup-table based, history table based, or based on someother function or combination of functions selected based on the circuitcomponents and their arrangement.

In another example of preferred methods for controlling driver circuits,additional data regarding parameters present in the circuit may also becollected and used to determine the appropriate set point for the drivevoltage. In the case of an LED system, for example, a light sensorintegrated into the system may be used to monitor the light level. Thisdata is used to adjust the LED drive voltage as needed to maintain thedesired brightness. Additional sensors, such as temperature sensors orimage sensors may also be used separately or in combination with thelight sensor to provide a more accurate set point. A light sensor usedas described may be included in close proximity to the LEDs orintegrated into the LEDs or LED arrays themselves. In suchimplementations, the light sensors may be used to measure the LEDoutput, rather than just the light returned to an external sensor. Inone implementation of this technique, a system using multiple diodes canuse one or more diodes as a sensing element while flashing one or moreof the remaining diodes. It is believed that the use of light outputdata enhances the precision and efficiency of the system. In an exampleof another possible variation among embodiments contemplated within thescope of the invention, in order to determine the turn-on voltage of theLEDs (or other load elements) a current can be applied to the diodeduring the supply capacitor charging. When the LED drive voltage reachesthe forward voltage of the LED, the LED begins to conduct and the LEDcathode voltage begins to increase. This point and one or moreadditional data points may be measured to extrapolate the diodecharacteristics, which may then be used to derive further useful datarelating to the operation of the system.

A further example of another possible variation useful for optimizingthe determination of the set point of the LED supply rail is to includeinformation from previous flash events. Using this technique, historicalvoltage and current for each flash event is measured and used to adjustthe set point. If the LED characteristics change over time, then themeasured changes can be used to make the appropriate adjustment everytime there is a flash event. This technique does not require additionalcalibration pulses, but nevertheless accounts for changes to circuitcomponents, such as LEDs, over time. The LEDs can be monitored andadjustments made for every flash event or for only some of the flashevents, depending upon the expected rate of change or other factors.

One example of such an implementation is to use factory settings to setthe initial voltage. During product test, the device is put throughseveral flash cycles and the initial LED drive voltage is therebydetermined. This information is stored in non-volatile memory and may beupdated, periodically or constantly, as needed during the life of thesystem as flash events occur. The non-volatile memory may beincorporated into the LED flash controller, or in a separate integratedcircuit or microcontroller associated with the system.

Again referring to FIGS. 2 and 3, additional system attributes may beused to enhance the implementation of the methods. For example, theefficiency of the system may be further improved by the incorporation ofan inductor in series with the diodes. The inductor allows forcontinuous current flow through the LEDs, precise regulation of thecurrent and full switching of the LED driver. An additional alternativearchitecture places the storage capacitors on the step-up converterinput. In this architecture, the converter control loop can control theoutput current directly; achieving high efficiency and highly accuratecontrol without a specific LED drive voltage set point calibration. Theexemplary step-up-converter shown can alternatively be a switchingregulator such as a boost converter or a charge pump. A resistor placedbetween the battery and the storage capacitors will limit the inrushcurrent and the current that is drawn from the battery during the flashevent. The diodes in this implementation can be placed in series and thedrive voltage increased as needed to provide the required current. Ifdiodes are placed in parallel, additional current control loops may beused to compensate for mismatch between the diodes.

To improve the quality of the captured image in a camera system, the LEDflash intensity can be changed during a single flash event and the imagecan be captured using more than one flash intensity. These multipleimages can be combined to eliminate effects resulting from too-high ortoo-low intensity, such as washed-out images. If several images areavailable that were captured using different light intensities, then thepixels that were overexposed at high intensity can be replaced with theinformation captured at low intensity. In an analogous fashion, pixelsthat were underexposed can be replaced by pixels that had a more optimumexposure. As a further improvement, the data collected at multipleexposure levels can be interpolated to achieve a more optimum image.This can be applied to exposure control or to red-eye reduction.

It should be appreciated by those skilled in the arts that theabove-described techniques may be used in various combinations toachieve a more optimum set point for a drive voltage. Analogoustechniques can also be used to set LED drive current using theabove-described methods. In using any of these techniques, additionalcalibration techniques such as a look-up-table or feedback can beincluded to account for temperature variation of the system. Any of thetechniques used to optimize the LED drive voltage for the flash eventcan also be used to optimize the system for a torch or lantern mode. TheLED voltage can be measured during the flash event or the history offlash events and can be used to measure the LED temperature. Thiseliminates the need for an external temperature sensor and can providethe exact LED temperature instead of just the temperature in theproximity of the diode. The methods of the invention provide one or moreadvantages including but not limited to improved performance and/orefficiency in driver circuits. While the invention has been describedwith reference to certain illustrative embodiments, those describedherein are not intended to be construed in a limiting sense. Variationsor combinations of steps or materials in the embodiments shown anddescribed may be used in particular cases without departure from theinvention. Although the presently preferred embodiments are describedherein in terms of particular examples, modifications and combinationsof the illustrative embodiments as well as other advantages andembodiments of the invention will be apparent to persons skilled in thearts upon reference to the drawings, description, and claims.

1. A method for driving a load comprising the steps of: determining adrive current requirement associated with the load; deriving a voltagelevel calculated to produce the drive current required by the load; andproviding the required drive current to the load using a low resistancedriver circuit.
 2. The method according to claim 1 wherein thedetermining and deriving steps further comprise characterization of oneor more load elements.
 3. The method according to claim 1 wherein thedetermining and deriving steps further comprise measuring equivalentseries resistance of a circuit for providing current to the load.
 4. Themethod according to claim 1 wherein the determining and deriving stepsfurther comprise characterization of one or more load elements in realtime.
 5. The method according to claim 1 wherein the determining stepfurther comprises providing one or more current pulses to the load; andwherein the deriving step further comprises using the one or morecurrent pulses to calculate the drive current requirement.
 6. The methodaccording to claim 1 wherein the determining step further comprisesproviding one or more low-current pulses to the load; and wherein thederiving step further comprises using the one or more low-current pulsesto calculate the drive current requirement.
 7. The method according toclaim 1 wherein the determining step further comprises extractingfeedback from the load for use in the deriving step.
 8. The methodaccording to claim 1 wherein the determining step further comprisesextracting feedback from the load for use in the deriving step, thefeedback comprising temperature data.
 9. The method according to claim 1wherein the determining step further comprises extracting feedback fromthe load for use in the deriving step, the feedback comprising lightoutput data.
 10. The method according to claim 1 wherein the determiningstep further comprises storing historical data relative to drivecurrent.
 11. The method according to claim 1 wherein the deriving stepfurther comprises retrieving stored historical data relative to previousdrive current.
 12. The method according to claim 1 wherein the loadcomprises one or more LEDs.
 13. The method according to claim 1 whereinthe load comprises one or more flash LEDs.
 14. The method according toclaim 1 wherein the voltage is supplied by one or more batteries. 15.The method according to claim 1 wherein the voltage is supplied by oneor more capacitors.